Remove lxml version restriction

Add documentation on how to avoid libxml2 version incompatibilities

Author: nosnilmot

README.md

@@ -88,6 +88,7 @@ Installation
 
  * python 2.7 // python 3.6
  * [xmlsec](https://pypi.python.org/pypi/xmlsec) Python bindings for the XML Security Library.
+ * [lxml](https://pypi.python.org/pypi/lxml) Python bindings for the libxml2 and libxslt libraries.
  * [isodate](https://pypi.python.org/pypi/isodate) An ISO 8601 date/time/
  duration parser and formatter
 
@@ -115,6 +116,14 @@ $ pip install python3-saml
 
 If you want to know how a project can handle python packages review this [guide](https://packaging.python.org/en/latest/tutorial.html) and review this [sampleproject](https://github.com/pypa/sampleproject)
 
+#### NOTE ####
+To avoid ``libxml2`` library version incompatibilities between ``xmlsec`` and ``lxml`` it is recommended that ``lxml`` is not installed from binary.
+
+This can be ensured by executing:
+```
+$ pip install --force-reinstall --no-binary lxml lxml
+```
+
 Security Warning
 ----------------
 

setup.py

@@ -34,7 +34,7 @@
     },
     test_suite='tests',
     install_requires=[
-        'lxml<4.7.1',
+        'lxml>=4.6.5',
         'isodate>=0.6.1',
         'xmlsec>=1.3.9'
     ],